1. Field of the Invention
The invention relates to coextruded, biaxially orientated films comprising a base layer, which comprises a polypropylene polymer or a mixture of polypropylene polymers, and at least one top layer comprising olefin polymers.
The invention furthermore relates to processes for the production of the film and to various uses of the film.
2. Description of Related Art
The films known from the prior art have the disadvantage that they have poor antistatic properties and have very poor destacking properties. In addition to their customary use as packaging film, films are also used as labeling material. For this, the film is cut into sections of suitable size, which are stacked on top of one another. During the labeling process, the individual cut section of film is removed mechanically from this stack and passed to the labeling process. Problems often occur with conventional films during this mechanical destacking operation when the machine frequently grips more than one sheet.
Furthermore, the conventional films have poor antistatic properties, which on the one hand leads to contamination of the surface due to accumulations of dust, but also, because of the large surface area of the film, represents a source of danger in the event of extreme charging, for example by sudden discharging in a printer, which may even lead to explosions of solvents.
The treatment, known in the prior art, of the base layer with antistatic agents improves the antistatic properties of the films insufficiently. Incorporation of corresponding additives into the top layer leads to problems due to evaporation during the production process. These films are not yet sufficiently antistatic either.
Opaque films are generally known. For example, EP-A-0 180 087 describes a five-layered sealable film which has a vacuole-containing base layer produced from propylene homopolymer and calcium carbonate, and an intermediate layer of propylene homopolymer and hydrocarbon resin, the sealable top layers of which film comprise propylene/ethylene copolymers. Because of the thick intermediate layer of polypropylene and hydrocarbon resin, the film has better mechanical properties and a higher gloss than films according to the prior art. In addition, the film can easily be corona-treated because of the sealing layers of propylene/ethylene copolymers. The film, however, is in need of improvement in respect of its antistatic properties and its destacking properties.
EP-A-0 312 226 discloses a multilayer opaque film in which a top layer comprises a readily printable polymer and which has an intermediate layer of propylene homopolymers and a core layer which contains vacuoles. The film is said to be distinguished by a good gloss when viewed from the homopolymer side. Because of the stated ratios in layer thickness, the surface gloss and the opacity or whiteness are in need of improvement.
EP-A-0 408 971 describes a film of high whiteness and high opacity, which is readily printable and is said to be glossy. The film has a cloudy structure, which probably originates from the small particle size of the calcium carbonate used. The particle size of 1.0 .mu.m stated as preferred results in a poor dispersability in the polymer. Furthermore, the increased fine content of the particles leads to a reduced opacity, since no vacuoles are thereby formed.
EP-A-0 234 758 and EP-A-0 225 685 describe readily printable opaque films. Because of the nature of the surface, however, the films are distinguished by a very matte appearance.